Solar radiation includes ultraviolet (UV) radiation, wavelength of which is between 200 nm and 400 nm. Exposure of skin to UV-A (320 to 400 nm) and UV-B (290 to 320 nm) causes various problems like reddening of the skin, localized irritation, sunburn, melanoma and formation of wrinkles. UV radiation is also known to cause damage to hair. Therefore, it is desirable to protect the skin and other keratinous substrates of the human body from the harmful effects of both UV-A and UV-B radiation.
Cosmetic compositions comprising sunscreen agents are used to protect the skin against UV radiation. The most commonly used UV-A sunscreen is of the dibenzoylmethane class. They are often used along with UV-B sunscreens to get wide spectrum sunscreen protection. It has been reported that stability of dibenzoylmethane compounds in sunscreen compositions is low when applied to the skin and exposed to solar radiation. The stability is poorer when an oil-soluble UV-B organic sunscreen especially from the class of cinnamic acid, is included.
Synthetic sunscreen stabilizers have been reported to solve this problem, e.g. as disclosed in EP 0514491 (L'Oreal, 1991) and more recently in US2009/039322 (Hallstar).
While the above references are directed to improving stability of sunscreen containing compositions, it is desirable to use stabilizers that are available in nature or extractable from natural sources like plants that the consumers consider to be milder and therefore is expected to have less irritation and allergenic potential when applied on the skin. Stabilisers which are available in nature or extractable from natural sources is also expected to be widely available and therefore can be provided to the consumers at low cost. Topical compositions comprising extracts from plant material are known for various purposes.
The present inventors have found that it is possible to enhance stability of the UV-A sunscreen of the dibenzoylmethane group by including certain compounds of the chalcone group.
U.S. Pat. No. 5,952,391 (Beiersdorf, 1999) discloses use of flavone derivatives and flavanone derivatives, in particular flavonoids for stabilizing cosmetic or dermatologically acceptable substances, the chemical formula of which includes the structural moiety of dibenzoylmethane, against the decomposition caused by UV radiation.
U.S. Pat. No. 4,867,964 (L'Oreal, 1989) discloses a cosmetic composition containing 2-hydroxylated chalcone derivatives and its use for protecting the skin and the hair against luminous radiations, new 2-hydroxylated chalcone derivatives and a process for their preparation.
These publications have not disclosed use of actives claimed in the present invention or natural extracts comprising the actives for stabilizing UV-A sunscreens.
It is thus an object of the present invention to obviate the drawbacks of the prior art and provide a highly photostable sunscreen composition while ensuring prolonged efficacy of the UV-A organic sunscreen used therein.
Another object of the present invention is to achieve the above object while additionally keeping costs low.